WEST DES MOINES, Iowa — Mitt Romney was all business, Rick Santorum was all smiles and Newt Gingrich pleaded for Iowans to punish negative ads as they fanned out across Iowa on Sunday searching for late momentum ahead of Tuesday’s caucuses.

Mr. Romney and Mr. Santorum got good weekend news — the final pre-caucus Des Moines Register poll showed Mr. Romney, the former Massachusetts governor, taking a lead, and showed Mr. Santorum, a former U.S. senator, in the midst of a surge that every day seems to catapult him closer to the top.

But in caucuses that will be decided by a little more than 100,000 hard-core voters, the men were taking nothing for granted and working for every last vote.

“This is the best part of politics,” Mr. Romney told reporters after campaigning at the Family Table restaurant in Atlantic, about an hour’s drive west of Des Moines.

Mr. Gingrich, who has seen a lead in Iowa slip away, said he regretted not responding earlier to a wave of attack ads against him.

But the former House speaker said he hopes Iowa voters punish the other candidates for their attacks — rehashing a theme that worked so well for 2008 GOP Iowa caucus winner Mike Huckabee.

“Iowans have a chance on Tuesday night to really change American politics,” Mr. Gingrich said while campaigning in central Iowa. “Business as usual would be for the negative ads to work.”

Mr. Santorum, who was not part of the wave of attack ads but had not shied from drawing sharp distinctions with his other primary opponents, on Sunday seemed to pivot to the same Obama-focused strategy that Mr. Romney has employed for much of the campaign.

“Denying our values, promoting values that are antithetical to everything we believe in — that is what this president is doing. He must be replaced,” said the former two-term senator.

For the most part, candidates Sunday didn’t make big policy news. But Mr. Romney on Saturday announced he would veto the Dream Act, which would provide a path to legal status for illegal immigrant students and young adults.

The Register poll, taken Tuesday through Friday last week, showed Mr. Romney leading with 24 percent support, Rep. Ron Paul in second with 22 percent, and Mr. Santorum third with 15 percent.

But looking at only the final two days, Thursday and Friday, Mr. Santorum grabbed 21 percent support and Mr. Paul slipped to 18 percent, suggesting an incredible shift toward the former senator from Pennsylvania.

This is a state where five of the six candidates actively competing have led in the polls at one time or another, and the only one who has never led a poll, Mr. Santorum, is seeing his support swell at exactly the right time as conservative voters continue to search for the anyone-but-Romney candidate.

The Register poll suggested that volatility remains: 41 percent of the likely caucus-goers surveyed said they could still change their minds. Mr. Gingrich said he thinks that number might be low.

In campaigning Sunday, Mr. Romney turned down a chance to attack Mr. Santorum but did point out that the senator backed Mr. Romney for the GOP’s 2008 nomination.

He also seemed to leave open the possibility that Mr. Santorum or another candidate would still win Iowa. Mr. Romney said he is prepared to run in contests across the country.

“I can’t tell you who’s going to win this thing, but I do believe I’m going to have a great deal of support and that will give me the kind of boost I need as I go into a season of other states,” he said.

Later in the day, in Council Bluffs, Mr. Romney packed an audience into an auditorium with a line snaking down the stairs, around a large reception room and out into the street an hour before the candidate was slated to speak.

Texas Gov. Rick Perry has been packing in the crowds, even better than the surging Mr. Santorum. On Thursday, Mr. Santorum drew a strong crowd to the Button Factory Restaurant in Muscatine, but an attendee who had been there for a Perry event earlier in December said the Texan’s audience was bigger.

Still to be seen, though, is whether Mr. Perry can translate those crowds into votes.

Walking out of a Santorum event in Marshalltown on Friday night, one woman was so wowed by the former senator that she confided to reporters she had a Perry sign in her yard but was going home to take it down and replace it with a Santorum sign.

Mr. Perry didn’t have any public events announced for Sunday, but on Saturday he continued to shift his focus from job-creating governor to outsider and dedicated Christian who would shake up the Beltway culture.

“I ask you to join me in echoing the words of the Prophet Isaiah: ‘Here I am, send me,’ ” he told voters in central Iowa.

Things had seemed much brighter for Mr. Perry when he entered the race in August and within a month jumped into a lead in the polls here, ending Rep. Michele Bachmann’s two-month reign. He even posted strong fundraising numbers through the end of September.

But catastrophic debate performances and controversial stances on illegal immigrants in Texas knocked him off his perch.

Still, Mr. Perry is rallying now.

The same cannot be said of Mrs. Bachmann, whose collapse has been the most spectacular because of the depths she has reached. Her Iowa campaign chairman renounced his support last week and backed Mr. Paul instead, and her events are drawing sparse crowds.

She finished what she called her 99-county tour and took the weekend off from stumping, according to the Register’s candidate tracker. She made just one stop to visit with volunteers at her campaign headquarters in Urbandale on Saturday and then attended services at Jubilee Family Church in Oskaloosa on Sunday.

She talked about the moment Oct. 31, 1972, when she was 16 and went to church at night. She said the Holy Spirit took a hold of her and she started pouring out her sins.

“I wasn’t a girl that had been involved in drinking. I hadn’t been involved in drugs or with boys, I hadn’t been out openly rebellious, but it didn’t matter. I still was a sinner,” she said. “All of a sudden my spiritual eyes were open and I could see. I wasn’t better than any other person. I was just free for the first time in my life.”

Appearing on the Sunday political talk shows, Mrs. Bachmann said with so many voters saying they are still persuadable, she is counting on a late burst of enthusiasm to carry her campaign.

“What we’re seeing going forward, especially with the tremendous outpouring of young people that are coming out to work on our phone banks and to go lit-dropping and door-to-door is nothing short of amazing,” she told ABC’s “This Week” program. “We’re No. 1 in the category of enthusiasm.”

That same claim, though, is also made by Mr. Paul, whose 2008 nomination bid earned fifth place in Iowa but who said the past four years have pushed voters toward him.

Mr. Paul, who led here in the polls briefly last month, took the weekend off from campaigning to go back to Texas, though he made the rounds of the Sunday talk shows and said he believes his supporters won’t desert him.

“I think the die has been cast,” he told “Fox News Sunday.” “Our numbers aren’t going to go down. The number of people, they’re not going to leave us, as they have on the other ones, they come and they go.”